Devices are known, which are designed to expand radially about a stage of turbine blades, so as to maintain a desirable clearance therebetween. A first example is described and illustrated in published patent specification 1484936. In that example, non rotating shrouds surround a stage of turbine blades. The downstream ends of the shrouds are hooked on a first expandable ring, which is located by radial dowels. The shrouds ends are also hooked in a ring of different expansion and contraction characteristics from those of the first ring. The upstream end of each shroud has an arm fixed thereto by one end, the other end having a ball thereon, which pivots in a socket in fixed structure when the first ring expands as a result of being heated, thus enabling, the first ring to lift the shrouds away from the tips of the blades. The other ring prevents too rapid movement of the shrouds towards the tips of the blades when cooling occurs.
A further example is illustrated and described in published patent specification 1605403. A turbine casing surrounds a stage of turbine blades, which again, include spaced, non rotatable shrouds. A polygonal member surrounds the turbine casing, and has radially arranged bolts fixed thereto so as to project radially inwards, towards the shrouds. The bolts heads locate in the opposing ends of expandable segments which surround the shrouds, which segments in turn, are hooked via their centre portions, to the opposing ends of the respective shroud segments. When the expandable segments are heated, they expand about their centres, into arched forms, thus lifting the shroud segments away from the tips of the blades.
Both examples of prior art disclosed hereinbefore rely entirely on expansion, and are comprised of a multiplicity of parts, which are extremely expensive to produce, and results in complexity of assembly. In the former example, there are provided valve mechanisms which themselves must be expanded, so as to enable heat to reach the shroud moving mechanism. In the latter example, accurate movement of the blade shroud segments about the pivot point of their respective arms, raises the need for, possibly, undesirably large clearances between their downstream extremities and structure adjacent thereto, and thus would reduce turbine efficiency through gas leakage.